Dear Dish-It

Ok so i dont do anything i try to not be around her but we share a room .. She is always taking my stuff, breaking my stuff, hitting me like not even a soft hit no like in the stomach and she hits very hard i tryed telling my mom abd dad they give her a talking to and she comes back to me and hits me for telling , she also blackmails me , and just so u know we are not talking about a 15 or 16 year old no we are talking about 20 years old she has been doing this for years now she dose not do this to my brothers i read online that people grow out of this but she did not also she is the oldest what should i do?